I'd say Wyoming or Montana if you're stateside, or Alberta if you want to go Up Nort'.
I hope you're not afraid of heights! And I hope you're both in great shape. You've got to climb a lot to even find the sheep, & then you've got to try to set up a stalk that make take all day... or even two!
There's definitely no need for a magnum caliber. Any reasonably flat-shooting cartridgefrom a .243 up through a .30-06 is fine, & even the .30-06 would be more than you'd really need. A lightweight rifle would certainly be your friend, but if you already own something else in an appropriate caliber, then use what you've got.
Here are a couple o' thoughts on the subject:
http://www.chuckhawks.com/mountain_rifles.htm
http://www.chuckhawks.com/hunting_rifles_game.htm
Best o' luck to you! It's a wonderful testament to your family that you guys would plan this.
FC